1. Field of the Invention
The invention relates to a magnetic field generating assembly for use in nuclear magnetic resonance (NMR) apparatus and, in a particular, to such apparatus for use in well-logging applications.
2. Description of the Related Art
It is well known that NMR can be used to inspect the geology around a bore hole. This is achieved by projecting a substantially uniform magnetic field into the rock surrounding the bore hole using a suitable field generator and then performing an NMR experiment within the projected region. Commercial well-logging tools make use of the Earth's magnetic field but this is not a satisfactory technique and is particularly unsuitable for inspecting the so-called "uninvaded zone" surrounding the bore hole. The uninvaded zone is that zone where there is no drilling fluid.
EP-A-0295134 describes apparatus and methods which seek to improve upon the earlier methods using the Earth's magnetic field. In this apparatus, four magnets are provided positioned with their north-south directions lying on a common axis and with the north-south directions of the outer magnets and inner magnets respectively being opposed. The magnets are fixed to one another for ease of movement along a bore hole. The electrical coil required to generate an rf magnetic field so as to perform the NMR experiment is mounted around the magnets.
This known arrangement suffers from a number of problems. Since there is magnetic material in the center of the apparatus, this reduces the field strength in the working volume projected outside the apparatus. Furthermore, positioning the rf coil around the magnets forces the designer to choose a non-conducting magnet material to avoid effects due to eddy-currents and noise coupling. This limits the magnet materials in practice to permanent magnet materials, such as ferrites, and so prevents the use of materials now available with up to three times the strength which would enable measurements to be made at greater distances into the rock. In addition, it is not feasible for producing large sensitive volumes because no provision is made for adjusting errors due to non-uniformities in the materials used and other differences between the as manufactured and designed arrangement.
An earlier approach is described in U.S. Pat. No. 4,350,955. In this case, just two magnets are provided spaced apart with their north-south axes aligned and with like poles facing one another. This arrangement has the advantage that the rf coil can be positioned in the space between the magnets with the consequence that the magnets themselves can be made of non-permanent materials. However, the placement of the magnets is intrinsically less efficient than in EP-A-0295134. Although the arrangement of U.S. Pat. No. 4,350,955 offers good vertical resolution (typically a few centimeters) it is not possible to change the radius of the working region in any convenient way. The ability to make measurements at several different distances into the rock is generally a requirement. Further, it has been reported that the apparatus of U.S. Pat. No. 4,350,955 can take 18 hours to measure one point. This is completely unacceptable in a practical well-logging situation.